A manipulator normally has a carrousel or turntable base whose upper part can pivot about an upright axis relative to its lower part and which carries a lower arm pivotal in turn on this upper part about a horizontal axis. An elbow defining another horizontal axis at the other end of the lower arm carries an upper arm whose outer end is provided with a so-called mechanical hand that iself can pivot about several axes on the outer end of the upper arm. This hand can open and close. Individual drives normally incorporating transmissions effecting substantial stepdowns are provided for each degree of motion, that is for motion about each axis, normally with drive motors directly at the respective joints. Obviously it is essential that such a device, which either is controlled by an operator from a remote location or operates independently under computer control, be able to work with great position. Thus the mechanical hand must be at the exact location where it is supposed to be. Any substantial deviation of the actual position of the hand from the desired position makes the manipulator virtually useless.
Hence it is necessary to provide playfree pivot drives, that is drives which have no lost motion. Attempts to make the various mechanical parts of the pivot drives to very tight tolerances have largely failed, as the net effect of a few normally inconsequential slippages can result in a substantial misplacement of the mechanical hand at the outer end of the upper manipulator arm.
Accordingly various playfree pivot joints and drives have been suggested (see Metallwissenschaft und Technik #12 1969, pp. 1289ff) to ensure exact placement of the mechanical hand. A typical such joint uses a pair of meshing gears with angled teeth. The gears rotate about parallel axes and one of the gears is biased by a spring axially into tight mesh with the other gear, so that full-contact meshing is ensured at all times. Obviously the friction and losses in such an arrangement are great.
Another playfree joint is described in Austrian patent No. 189,469 of Apr. 10, 1957. Here a complex shaft arrangement incorporating a torsion rod and an outer tube shaft extends between rotatable and nonrotatable gears in a manner to substantially eliminate play. Once again the system has substantial backlash and friction.